356 AXNT20 KNHC 161723 TWDAT Tropical Weather Discussion NWS National Hurricane Center Miami FL 1805 UTC Thu Sep 16 2021 Tropical Weather Discussion for North America, Central America Gulf of Mexico, Caribbean Sea, northern sections of South America, and Atlantic Ocean to the African coast from the Equator to 31N. The following information is based on satellite imagery, weather observations, radar and meteorological analysis. Based on 1200 UTC surface analysis and satellite imagery through 1630 UTC. ...SPECIAL FEATURES... Post-Tropical Cyclone Nicholas is centered near 29.8N 91.7W at 1500 UTC or 100 nm SSE of Alexandria Louisiana moving N at 2 kt. Estimated minimum central pressure is 1007 mb. Maximum sustained wind speed is 20 kt with gusts to 25 kt. A trailing trough is analyzed over the Gulf of Mexico and extends SW to near the Texas coast along 27N97W. Nicholas is currently nearly stationary along or just offshore of southern Louisiana, and recent visible satellite imagery suggests the center is attempting to reform just to the SW. However, this low is expected to gradually move northward over the next couple of days. Shallow low to middle level clouds and minimal convection is noted near the center of the former Nicholas at this time. However, scattered moderate to strong convection persists to the east and south of the trailing trough, from the Florida Panhandle to the Mexican coast along 98W, where fresh southerly low level winds are converging along and just ahead of the trough. The convection will shift with and ahead of the remnants of Nicholas during the next few days, and is expected to produce additional rainfall amounts of 2 to 4 inches across the central Gulf coast Friday, with isolated storm totals of 12 inches possible. Flash flooding impacts, especially in urban areas, are possible across these regions. Widespread minor river flooding is expected, while scattered moderate river flooding is possible, across portions of southeastern Louisiana, southern Mississippi, southern Alabama, and the Florida Panhandle. The Weather Prediction Center is issuing bulletins on Nicholas and they can be found at http://www.wpc.ncep.noaa.gov. A surface trough extends from broad 1011 mb low pressure located near 31.5N74W to just north of the Bahamas near 28N77W. Scattered moderate to strong convective activity associated with this low is displaced to the E of the trough and covers the area from 24N to 35N between 64W and 73W. Fresh to strong winds associated with this system are also noted to the E of the low center and mainly from 28N to 31N between 68W and 74W. Environmental conditions are expected to become more conducive for development, and a tropical depression is still likely to form during the next day or two while the system moves north- northwestward to northward off the southeast U.S. and mid Atlantic coasts. Regardless of development, this system could bring high surf to portions of the southeast and mid- Atlantic U.S. coasts later this week. This system has a high chance of tropical cyclone formation in the next 48 hours. Please read the latest Tropical Weather Outlook at www.hurricanes.gov for more details. A tropical wave is in the Atlantic Ocean along 34W from 03N to 17N, moving west at 15 to 20 kt. Associated showers and thunderstorms are slightly less organized with this wave this morning. However, environmental conditions are still expected to remain conducive for development, and a tropical depression is likely to form during the next couple of days. This system is expected to move westward to west-northwestward across the tropical Atlantic during the next several days. This system has a high chance of tropical cyclone formation in the next 48 hours. Please read the latest Tropical Weather Outlook at www.hurricanes.gov for more details. ...TROPICAL WAVES... Refer to the Special Features section above for details on a tropical wave and associated area of low pressure in the Atlantic Ocean along 31W/32W. Another tropical wave is emerging off of the west coast of Africa this morning, and will be added to the 1800 UTC surface analysis. Satellite imagery and surface observations suggest that there is a 1012 mb low along the African coast near 13.5N17W, shifting westward. The Dakar, Senegal 1200 UTC sounding also suggests that the wave is nearing the coast. Morning scatterometer winds showed a broad NE wind surge of around 20 kt spreading from the coast of Mauritania southwestward to near 30W. Environmental conditions are forecast to be conducive for development while the system moves generally west- northwestward to northwestward over the far eastern Atlantic. Currently, a large cluster of moderate to isolated strong convection is noted from 05N to 13N and E of 25W to the coast of Africa. A tropical wave is in the Atlantic Ocean along 47W, from 04N to 14N, moving west at around 10 kt. Scattered showers are noted where the wave meets the monsoon trough near 07.5N47W. A tropical wave is in the Caribbean Sea along 75W from 18N southward to Colombia, moving west at 10 to 15 kt. The tropical wave combined with a mid to upper level trough axis across the western Caribbean is aiding to induce scattered showers and thunderstorms along the trough axis from 10N to 12N between Colombia and far SE Nicaragua. A tropical wave is over Central America along 90W from the central Yucatan Peninsula southward into the eastern tropical Pacific Ocean, moving west at 10 to 15 kt. Scattered showers and a few thunderstorms are noted over the E Pacific. ...MONSOON TROUGH/ITCZ... The monsoon trough emerges off the coast of Africa through Senegal to a 1012 mb low along the coast near 13.5N17W to 08N22W to 1013 mb low pressure located near 08.5N39.5W to 06.5N46W. The ITCZ continues from 06N47W to the coasts of French Guiana and Suriname. Aside from the convection mentioned above near the tropical waves, scattered showers and moderate thunderstorms are noted from 09N to 12N between 54W and 61W. GULF OF MEXICO... The Weather Prediction Center is issuing bulletins on Post Tropical Nicholas, located along the coast of southern Louisiana. Refer to the Special Features section above for details. As previously mentioned, a surface trough extends from the cyclonic circulation of Nicholas to near 27N97W. Morning scatterometer winds showed gentle to moderate SE-S winds east of 88W, while moderate to fresh S winds were detected between 88W and the trough. Gentle to moderate NW-N winds prevail west of the trough toward the Texas coast. Seas are generally 2 to 3 ft across most of the basin, except 3 to 5 ft over the north- central Gulf where fresh southerly winds are occurring to the east of the circulation of Nicholas. Scattered showers and thunderstorms, some strong, are noted within 180 nm S and SE of the trough described above, and most numerous off of the Mexican coast. Post-Tropical Cyclone Nicholas is nearly stationary along the central Louisiana coast. Moderate to fresh winds with seas of up to 5 ft may prevail in the north central Gulf into tonight as a result. Elsewhere, gentle to moderate southerly winds will dominate into the weekend. CARIBBEAN SEA... Refer to the Tropical Waves section above for details on a tropical wave moving across the Caribbean Sea. Morning scatterometer winds showed fresh easterly winds across the central Caribbean between the tropical wave along 79W and 70W, and fresh SE winds across the outer Gulf of Honduras to just offshore of Cozumel, Mexico. Gentle trade winds prevail elsewhere. Seas are generally 2-4 ft, except 5-6 ft in the areas of strongest trade winds are blowing. Showers and thunderstorms continue to flare-up over the NW Caribbean due to a combination of a tropical wave moving westward across the basin and an upper-level low trough currently extending SW across the western basin. A tropical wave across the central Caribbean will move across the western Caribbean into the weekend. Another tropical wave will move across the Lesser Antilles this weekend. Strong winds will pulse in the overnights into early next week offshore Colombia and in the Gulf of Honduras. Low pressure in the eastern Atlantic is likely to develop into a tropical depression over the next couple of days, and this could bring increasing winds and seas to the waters near the Leeward Islands for the start of next week. ATLANTIC OCEAN... Refer to the Special Features section above for details on the disturbance situated northeast of the Bahamas, and the system in the eastern Atlantic. Both systems have a high chance of tropical cyclone formation through 48 hours. A ridge along about 36N dominates the Atlantic forecast waters E of 65W, with the main high pressure center of 1031 mb just south of the Azores. A surface trough continues to move westward across the tropical Atlantic and extends from 26N37W to 17N39W. A cluster of scattered showers and a few thunderstorms is noted over northern portions of the trough. Scatterometer data depicted very well the wind shift associated with the trough axis. Moderate to fresh NE-E winds are noted east of the trough to offshore of Africa. Seas are in the 6 to 8 ft range across this area. West of 65W, morning scatterometer winds depicted gentle moderate winds between the Bahamas and 65W, except fresh southerly winds north of 28N between the trough and 65W. Seas are generally 3 to 5 ft except 5 to 7 ft north of 27N in the fresh southerly wind flow. Showers and thunderstorms are increasing in association with low pressure several hundred miles SSE of the Outer Banks of North Carolina. A tropical depression is likely to form, however this system is moving northward out of the basin, so any increased winds and seas will be north of the area by Fri. Elsewhere, weak high pressure will lead to gentle to moderate mainly easterly winds across the basin. Another low pressure may impact waters north and east of the Leeward Islands early next week. $$ Stripling